p1126 & p1133 adventures - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Hi folks. I wanted to share my experience with DTCs p1126 and 1133, in the hopes that this helps someone else with similar issues. I know a lot of this has been covered elsewhere, but lots of threads trail off with no conclusion or confirmation of success or failure. A couple weeks ago I did a ~1600 mile road trip in my 2002 S 6spd, to the SF Bay Area from San Diego. I've driven my share of quirky/old/german cars, so totally expected to have some sort of mayhem. Driving up the scenic route via US-101 was a breeze. After getting to my first stop at my in-laws house in the Half-Moon-Bay/Coastside area, I found I'd damaged my clutch hydraulic line, just aft of the left-front wheel well. I'm still not sure what I ran over, but I blame the 405 freeway. That was trivial enough to sort out enough to finish my trip and get home. Also my rubber windshield trim blew out on the freeway, and with the top up it sounded like I had a possessed speaker amp! The final day of my trip (a 615-mile trek from Petaluma to Prunedale to SD), the car was hard to start. Lots of cranking before it would fire, but the tach was moving so I knew it wasn't a dead crank sensor. It would eventually fire, and ran without issue. But naturally, after my last fuel stop in Santa Maria, the CEL popped up, p1126 and p1133 (lean, max enrichment pegged on both banks). Somehow I'd left the Durametric at my in-laws house, and only had my little Ancel code reader. So, I erased the codes, the car ran like trash for a minute, and then evened out and the light stayed off for a bit. I made it through orange county before the light came back on just getting into Carlsbad. It was late, and I limped home. The car made a few pops while rev-matching getting off the freeway. When my Durametric showed up in the mail a couple of days later, I went through all the usual business of looking at fuel trims and looking for intake leaks. I found a few things that could have easily explained high LTFT at idle (RKAT): Loose intake boots on the resonance tube, enough that it would shift visiably under throttle application Very leaky vacuum check valve ahead of the secondary air-injection vacuum lines A cracked vacuum line to the electronic changeover valve that connects to the resonance tube flapper Leaky bearing seals on the resonance tube flapper After fixing most of that, crankcase vacuum was around 5.5" - 6" h2o, and intake at 19-20 in-Hg. So, that dropped my RKAT on both banks from around 5, down to 2-ish. Still up, but not maxed out. FRA was still sitting at 1.31. No good. Went for a drive with the durametric hooked up and the laptop on the passenger seat. Trims would stay fairly normal while driving around the neighborhood... but as soon as the car was warmed up, FRA would shoot right back up to 1.31. Lumpy idle, uncooperative starting. Finally, I checked the fuel pressure: not quite 30psi running, then the car died, and I watched the needle on my gauge fall swiftly to 0. Then I tested the fuel pump relay on the bench... it worked fine, opened and closed normally. I jumpered the fuel pump and tested again. This time, 25psi running. Then 20. Then 18! I was watching my pump crap out in real time! After a new pump and a dozen or so miles, I'm seeing much more acceptable fuel trims. The car feels fantastic now! If the fuel trims stay this way, I think I'm set... right? I've been driving this car for a little over a year now, and I'm really enjoying this wealth of knowledge and information on this forum. It seems like most things that can go wrong have already gone wrong and been well documented and discussed! I was tempted a few times to fire the parts cannon, but I held back and kept searching and testing... and learning.

Nice write ✍️ up. Hopefully that sorts it all out for you. It’s amazing how many things affect fuel trims. Mine are usually a bit different for bank 1 and bank 2.

LN's video with Tony Callas talking about fuel trims was very helpful: https://www.youtube.com/watch?v=HW7BmBIJDXs He's pretty clear that fuel trims at idle (RKAT) are typically vacuum leaks, and trims off at cruise (FRA) are usually fuel delivery issues. This thread by Ike84 also helped a ton, and I'm usually better with long forum posts than I am with youtube vids: https://986forum.com/forums/boxster-general-discussions/81522-causes-lean-conditions.html#post640946 I definitely fell victim to thinking that the car was rich. Lots of things cause lean with these cars, which makes the DME go rich trying to compensate. My car also had intake backfires, gassy rich smelling exhaust, and high fuel consumption, and in the VW days I'd look at that and say it was rich. Well, the DME knows the data, and when you make it spill the beans the truth becomes clear. I put some more miles on the car this morning. The trims still look pretty good!

Great write up, really appreciated!